During the manufacture of an automobile at an assembly plant, end of line tests are used as quality control measures. Typically, end of line tests include a first start as an overall engine system check. The time to complete a first start is critical for maximizing production capabilities and producing cost competitive products. The fuel priming step in the assembly process is used to fill the fuel lines and fuel rails of the fuel system with fuel so that the first start of the vehicle will be quick and efficient. Without priming the fuel system, the starter must turn the engine for an unnecessarily long time while the fuel system purges trapped air before delivering fuel.
Conventional fuel priming methods used for fuel delivery systems containing a return line simply turn on the fuel pump for a certain time interval. Turning on the fuel pump delivers fuel up to the fuel rail and trapped air is sent back to the fuel tank via the return line. Thus, when a vehicle is first started after priming, the start time is minimized. However, incomplete fuel system priming occurs when the vehicle contains a returnless fuel system. Returnless fuel systems do not contain a return line or relief valve which, when using conventional priming methods, are necessary for releasing trapped air during the fuel priming step of the assembly process. Thus, during a first start, the starter must continually turn the engine until the trapped air is passed through the cylinders and exhaust system, resulting in significant starting delays. Priming methods are known where trapped air in a fuel delivery system is vented into the engines combustion chamber. One method uses a bypass valve to allow trapped air to circumvent a normal flow path and enter the engine burn chamber. Such a system is disclosed in the U.S. Pat. No. 4,449,359.
The inventors herein have recognized numerous disadvantages with the above approaches. One disadvantage is the necessity of additional hardware to create an alternate flow path as in the case of U.S. Pat. No. 4,449,359. Using this system for vehicles is disadvantageous because the cost of additional hardware to solve a problem that only occurs during the first start at an assembly plant is unacceptable. A second disadvantage is that circumstance may occur where fuel is accidentally injected into the engine in an attempt to vent trapped air. Accidental injection of fuel can cause the vehicle to experience a long first start, can cause the vehicle to fail end of line emissions tests, and can cause hydraulic locking of the engine requiring costly and time consuming repairs.
Consideration of a system that monitors absolute fuel pressure in an attempt to assist in the control of venting trapped air has been made. However, the inventors herein have recognized that this approach is susceptible to the problems previously described. For example, in typical fuel delivery systems absolute pressure does not have any correlation to the amount of air trapped in the fuel delivery system. Further, systems which monitor absolute pressure do not provide any information to determine the proper amount of purge time.